Ink-jet printing involves projecting a stream of ink droplets on to a substrate, such as paper, to form the desired image. Although ink compositions for ink-jet printing are generally based upon water-soluble dyes, the solubility of the dyes results in less than optimal image resolution.
Most soluble dyes are soluble in water or in a mixture of water and a water-miscible solvent. When an ink composition comprising a soluble dye is used with ink jet printers, various problems arise from the tendency of the solvent, along with the dissolved dye, to migrate beyond the site of deposit. Effects associated with solvent migration include "feathering" from the overlapping of adjacent dots of ink to "print through" from the penetration of the solvent and dye into the paper. Although print resolution may be improved by using special paper coated with materials to prevent the ink from either spreading or soaking through, the special paper increases the print costs and thus is not widely used.
Unlike water-soluble dyes, pigments are substantially insoluble and remain as particles in aqueous solution. Because the pigment particles do not necessarily migrate with the solution, many of the problems associated with water-soluble dyes are avoided. Moreover, when compared with soluble dye based ink compositions, pigment based ink compositions tend to have superior waterfastness, lightfastness, image density and thermal and oxidative stability. Unfortunately, unmodified pigment particles have a tendency to agglomerate. So instead of remaining stably suspended over time, pigment particles eventually settle out of solution clogging printer jets and/or resulting in images that appear faded.
Various solutions to agglomeration have been tried including surface modification, use of dispersants, and encapsulation of pigments in polymer coats. However, because these solutions tend to be laborious, expensive, and need to be individually tailored to each pigment formulation, pigment based ink compositions are not widely used in ink jet printers. As a result, a need exists for simple and low cost ink compositions that are capable of producing high resolution images using ink jet printing methods.